In recent years, in relatively small size batteries except for currently used dry cells, button-type cells, and coin-type cells, a spirally-rolled electrodes with separator enabling the high-rate discharge by expanding the surface area of both positive and negative electrodes is often employed as a means to construct electrodes, regardless of whether the batteries are primary or secondary.
However, since a relatively high-rate discharge and rapid rechargeability are required at the same time in battery market generally, most of the secondary batteries are employing the construction of spirally-rolled electrodes. Therefore, the way to comprise this construction is employed for most sealed cylindrical batteries or sealed prismatic batteries of nickel-cadmium batteries (Ni/Cd batteries), nickel-metal hydride batteries (Ni/MH batteries), and lithium ion batteries (Li-ion batteries) which represent secondary batteries. In other explanation, this spirally-rolled electrodes are employed for over 55% of the total battery production in currency in Japan. Therefore, the explanation goes ahead focusing sealed small size secondary batteries as an example.
With many battery systems selected naturally, by around 1990, the secondary batteries with the industrial production scale have been limited to lead acid batteries and Ni/Cd batteries. Particularly, the latter battery systems have remarkably expanded the market with the rapid, widespread use of the portable electronic devices including CDs and camcorders in 1980s, by being used as a power source. The main reason for this is that Ni/Cd batteries have higher energy density than lead acid batteries, which means that the former is more suitable for small-size and lightweight.
However, with the advent of 1990's, subsequent to the Ni/MH batteries with high volumetric energy density, Li-ion batteries with less weight have been developed, both of which have come to enter the market of Ni/Cd batteries. As for the shape of the battery systems, in the case of Ni/MH batteries, small size sealed cylindrical batteries are mainly used as in the case of Ni/Cd batteries. In Li-ion batteries, both small size sealed cylindrical batteries and sealed prismatic batteries are used.
In addition to the aforementioned purposes, Ni/MH batteries with larger size than the batteries for the said purposes, yet still classified to small size sealed cylindrical types, have come to mainly be used with the expansion of the power source market including hybrid electric vehicles (HEVs), electric-assisted bicycles, and the like.
As for the structure of the electrodes in these batteries, sealed cylindrical batteries for Ni/Cd batteries, Ni/MH batteries and Li-ion batteries are basically the same. It is the structure of electrodes wherein a positive electrode with a thin sheet of a rectangular plate and a negative electrode with a thin sheet of a rectangular plate are wound spirally depicting a substantially concentric circles with a separator made of a synthetic resin therebetween.
Further, although sealed prismatic batteries of Li-ion batteries have basically the same structure of electrodes with a spiral shape, the spirally-rolled electrodes with separator have nearly a flat elliptical shape, instead of a concentric shape. The said electrodes are inserted into the prismatic case with one side closed.
For information, both sealed prismatic Ni/Cd batteries and Ni/MH batteries employ the structure that several positive electrode plates and negative electrode plates are stacked alternately with a separator. However, this type has the low market share at present. In other words, regardless of the battery systems, the spiral type is the main structure of their electrodes in small size batteries.
To explain the prior art of this structure of spirally-rolled electrodes more specifically, a sealed cylindrical Ni-MH battery system is focused as an example.
Ni/MH batteries are such battery systems wherein nickel oxide powder (mainly oxy-nickel hydride) is used as the positive electrode material, and hydrogen absorbing alloy such as MmNi5 alloy system is used in the negative electrode as a pseudo-active material respectively, whose battery voltage per cell is about 1.2V. The pseudo-active material used in the present invention is the material that is capable of absorbing or desorbing the active material including hydrogen or lithium during an electrochemical reaction. The active material that is occuluded in pseudo-active material is desorbed during the discharging, and is absorbed during the charging, as an active material or a compound of the active material.
These active materials or pseudo-active materials are filled in the three dimensional substrate or coated on the two dimensional substrate with a conductive material, a binder and the like. As required, the substrate that is filled in or coated on may be pressed, which is the thin electrode for the battery system.
The electrodes are obtained by winding spirally with the positive electrode and the negative electrode interposing the non-woven hydrophilic polyolefin separator therebetween. Subsequently, the electrodes are inserted into the cylindrical metal case with one side closed, in which alkaline electrolyte is poured, thereafter sealed by the cap, acting as a thin plate positive electrode terminal and safety vent, with a gasket made of synthetic resin, thereby obtaining a battery. Here, in this battery, like in general Ni/Cd batteries and Li-ion batteries, the battery capacity is regulated by the charging and discharging capacity of the positive electrode. In other words, the distribution of the battery capacity practically corresponds to the distribution of the weight of active materials filled in or coated on the positive electrode substrate, although there do exist some errors due to the distribution of the utilization of active materials.
As aforementioned, since small size secondary batteries have originally been used for high-rate discharge, the structure of the spirally-rolled electrodes has been required. Further, in recent years, as for Ni/MH batteries, with the development to the market of power use which requires further high-rate discharge performance, the structure of the spiral electrode group using thinner and longer electrodes than the conventional electrodes has been drawing attention.
In addition, for this purpose, since the battery is used at a high voltage of several hundred volts, much higher voltage than for the consumer use, in the batteries with 1.2 V systems, many cells are used in series. For example, in the batteries for HEVs, which have been mass-produced recently, 240 cells or 120 cells of D-size Ni/MH batteries are used in series.
In the said case, batteries with a small capacity are susceptible to the damage of overdischarge or overcharge as well as limit of the capacity of whole battery system in series, which sometimes makes the whole battery capacity low, and shortens the cycle life as well. It can be said that this is a fatal problem, particularly for this purpose. Therefore, to keep the distribution of the battery capacities narrow has become a more significant issue than ever for obtaining the reliability of the power source.
Therefore, in sealed cylindrical Ni/MH batteries, particularly in the positive electrode which determines the battery capacity, the following three studies for the improvement have been conducted for reducing the distribution of the capacities among batteries.    1. To improve the technique or apparatus to reduce the distribution of filling or coating of the active material powder.    2. To measure the weight of all the electrodes and select such electrodes as having nearly the same weight.    3. To measure the discharging capacities of all the batteries and select such batteries as having nearly the same discharging amount.
As aforementioned, since higher-rate discharge is necessary for power use compared with the consumer use, thinner electrodes are used usually. For this, due to the expansion of electrodes caused by repeating charge and discharge cycles, a part of the electrode is prone to deform concentric circle shape or to deform flat elliptical shape of the spirally-rolled electrodes. And new risk has emerged that, under some circumstances, the microscopic short circuit may occur between the both electrodes with a separator broken through by a deformed portion of the electrode. Here, this risk is the phenomenon also recognized in the Li-ion batteries that already have developed the thin electrodes.
In other words, in Ni/MH batteries or Li-ion batteries using the structure of the spirally-rolled electrodes, when a longer and thinner electrodes which are weaker mechanically than the conventional electrodes are used as a means to make batteries of high-power, the partial distortion from concentric circle or original ellipse of the spirally-rolled electrodes becomes remarkable caused by the expansion of the electrode with the increased charge and discharge cycles, and under some circumstances, the batteries often have the risk of microscopic short circuit.
By the improved measures mentioned in the said study 1, aiming to reduce the distribution of the said battery capacity, a certain amount of the active material powder filled or coated per area whose main material is Ni(OH)2 can be achieved with considerable progress. For example, the original variation of ±7˜10% is reduced to around ±3˜5%.
However, it is not enough for the use of a power source for HEVs which uses a few hundred cells in series.
By the improved measures mentioned in the said study 2, by providing the ranking by weight, the amount of active material filled in a certain weight range can be known, and the battery capacities can also be known in a certain range by using the electrodes in the same class. However, the capacity distribution of whole cells cannot be reduced.
The improved measures mentioned in the said study 3 has the similar concept to that mentioned in the said study 2 and is more accurate since the battery capacity is measured directly in the improved measures mentioned in the said study 3. However, in order to obtain accurate capacity of the battery, several cycles of the charge and discharge are required, which is complicated and troublesome. Further, the capacity distribution cannot be reduced as a whole cells as in the improvement measures mentioned in the said study 2. In addition, as for the prevention of microscopic short circuit, no new measures for solution have yet to be reported.